1. Field of the Invention
This invention relates to instruments for the noninvasive quantitative measurement of constituents in blood, such as blood glucose levels. Specifically, this invention relates to an improved analysis instrument which achieves improved accuracy in measurement by ensuring that the position of the subject's finger in the instrument remains constant each time the finger is inserted into the instrument.
2. Description of the Background Art
Information concerning the chemical composition of blood is widely used to assess the health characteristics of both people and animals. For example, analysis of the glucose content of blood provides an indication of the current status of metabolism. Blood analysis, by the detection of above or below normal levels of various substances, also provides a direct indication of the presence of certain types of diseases and dysfunctions.
A current type of blood glucose analytical instrumentation is available for the specific purpose of determining blood glucose levels in people with diabetes. This technology uses a small blood sample from a finger poke which is placed on a chemically treated carrier and is inserted into a portable battery operated instrument. The instrument analyzes the blood sample and provides a blood glucose level reading in a short period of time.
A different class of blood glucose analytical instruments is the near-infrared quantitative analysis instrument which noninvasively measures blood glucose, such as the type described in U.S. Pat. No. 5,077,476, incorporated by reference herein. The noninvasive blood glucose measurement instrument analyzes near-infrared energy following interactance with venous or arterial blood, or transmission through a blood-containing body part. The instrument measures a change in light absorption that occurs, in part, due to the glucose content of the blood stream.
Non-invasive measurement instruments of this type have broad applications for the diabetic community. For example, people with diabetes have wide changes in their blood glucose content during the day which often require multiple measurements per day for good disease control. The ability to make these near-infrared blood glucose level measurements noninvasively means that more measurements will likely be made per day than would be made using the more painful blood drawing approach.
An example of a non-invasive measurement instrument is disclosed in U.S. Pat. No. 5,086,229, also incorporated by reference herein, wherein an individual user places the most distal portion of his or her finger within a "jaws" type arrangement. Near-infrared energy within the spectrum of interest is then impinged upon the surface of the finger and a detector is placed axially with the near-infrared beam on the opposite side of the finger to receive any near-infrared energy emerging therefrom. A microprocessor receives the amplified signal from the detector and calculates the user's blood glucose level. The near-infrared energy is within a bandwidth of 600-1100 nm, and preferably 600-1000 nm.
Data obtained from experimentation has revealed that the accuracy of such non-invasive measurement of a blood analyte such as glucose is dependent upon the repeatability of proper finger positioning within the instrument. In particular, the inserted finger must be consistently aligned with the optical axis of the instrument. The light path of the near-infrared light source should preferably pass through the center of the cuticle to obtain high accuracy measurements. Thus, it is desirable to avoid rotation of the finger once inserted into the instrument.
There is a natural tendency when grasping an object for the index finger and thumb to form a pincer-type relationship in which the index finger is slightly rotated with respect to the other fingers so that the finger pad on the bottom side of the index finger faces the pad on the bottom side of the thumb.
FIG. 1 illustrates a typical near-infrared measurement apparatus 10 for non-invasive optical measurement of blood analytes. The apparatus has an aperture (not visible) for insertion of the index finger of the hand 20 of a subject into the optical path of the light source between the light source and detector. In this design, the left index finger is inserted into the aperture of the unit with the thumb on one side of the unit and the other three fingers on the other side of the unit. As such, the index finger tends to rotate upon insertion to form a pincer-type relationship with the thumb. Dependent on the particular positioning of the thumb and the other three fingers on the unit during any one insertion, the index finger will assume a different rotational placement within the light path of the instrument.